Substantially all current microlithography tools include a device, called an “autofocus” device, used for achieving a specified sharpness (“focus”) of images as produced on the surface of lithographic substrates. A conventional autofocus (AF) device automatically monitors position of the lithographic substrate along an axis (conventionally the z-axis or height axis) relative to the imaging optical system. The positions are usually determined based on a detected position of a beam of light reflected from the substrate surface. When the position data are obtained between exposures, the AF sensor must be very fast so as not to reduce throughput. Because of this speed requirement, AF sensors are usually “optical,” usually involving the direction of a beam of light onto the surface of the substrate obliquely, at a glancing angle of incidence (for example, an angle of incidence greater than 80°). The position of light, reflected from the substrate surface, as incident on a detector corresponds to the z-position of the substrate. The light beam is usually incident on the substrate near the intended imaging site of the lithographic exposure. The substrate is coated with a layer of photoresist prior to exposure. Using the top surface of the resist as a convenient focal plane reference, an image is produced within the thickness of the photoresist layer.
Besides optical AF devices, other conventional devices for sensing substrate position are “physical” devices, such as an air gauge or capacitive gauge. Each of these types of devices has respective advantages and disadvantages. One type of air gauge comprises a gas-discharge port placed in proximity to the substrate surface. In the air gauge the port is coupled to a source of compressed air or other suitable gas. The gas discharged from the port changes pressure as a function of the nearness of the substrate surface to the port. Monitoring the pressure provides a detection of the corresponding distance of the substrate surface from the port (e.g., the vertical position, or “height”) of the substrate. Air gauges can be very accurate, but they produce height data slowly when compared to certain types of optical AF sensors. Also, the air flow discharged from the air gauge can have an undesired thermal effect on the substrate and/or can be a source of contamination of the substrate.
Another device useful for measuring substrate position is a capacitive gauge (called a “cap sensor”), of which a small capacitive probe is placed near the substrate surface. The capacitance measured by the probe changes as a function of the position of the substrate (e.g., height of the substrate relative to the probe). Cap sensors can be adversely affected by electrical properties of the films and features on the substrate.
Optical sensors are easily configured to obtain many measurements at many locations on the substrate in parallel, thereby producing a large amount of data very rapidly. A cap sensor can be very fast when measuring a single point, but scaling the measurement speed to an entire substrate (e.g., a 450-mm diameter semiconductor wafer) is difficult to achieve.
Whereas physical sensors such as air gauges are accurate, they are too slow for practical use in high-volume manufacturing. In contrast, optical sensors are extremely fast, but can be adversely affected by the optical properties of the substrate surface and underlying structures, such as layers (usually but not necessarily patterned) already formed on the substrate during earlier process steps. Light reflected from the substrate frequently exhibits changes in intensity and/or phase that are largely unrelated to actual substrate height. An optical sensor may sense errors related to these changes, particularly changes that are non-linearly related to the thicknesses and refractive indices of the underlying layers. These errors are termed “AF errors” or “focusing errors.” One source of AF errors arises from some of the AF light (which may form images of “slits” or “fringes” on the substrate) reflecting from previously formed patterned thin films beneath the surface of the substrate. The magnitude of AF errors of this general type can vary with the particular pattern(s) and other features in the previously formed layers on the substrate, and can vary with the thickness profiles of those layers. These AF errors can be substantial.
An exemplary AF error arising from the presence of thin-film layers previously formed on the substrate surface arises from the Goos-Hänchen (GH) effect, in which a substrate surface including thin films formed during earlier process steps produces a shift or offset in position of a beam of light reflected from the surface. This shift is not related to an actual change in position of the substrate, but can be mistaken for one. Patterning in previously formed thin films is not required for the GH effect to occur (but patterning can be a factor). Rather, the simple presence of the previously formed thin films is required. If the substrate has the same thin-film stack applied uniformly over the substrate surface, then the result usually is a substantially uniform GH effect and produces a uniform offset that is easily treated by introducing an offset from the measured substrate height. Otherwise, GH effects can vary appreciably over the substrate surface, depending upon regional variations of thickness and other parameters of the thin films over the surface as well as pattern variations from one region to the next. For example, a change in regional offset due to the GH effect can arise in an area containing memory relative to another area containing logic. Also, different patterns, although they may consist of the same material, may produce different offsets from GH effects by virtue of the structures, spatial frequencies, orientation, duty cycles, etc., of the respective patterns relative to other patterns.
In producing a GH effect, previously formed layers on the substrate surface can change the intensity of the reflected AF light and/or the phase of that light. According to one way of looking at the GH effect, a monochromatic AF beam incident on a reflecting surface is decomposed into multiple plane waves. The reflective surface (i.e., the substrate surface) produces a different phase for each of these plane waves, depending on the wave's angle of incidence. Over a small range of incidence angles, corresponding to a converging or diverging wavefront, the phase of reflection can either increase or decrease with the angle of incidence, which produces a tilted wavefront in the far field corresponding to a physical shift of the beam in the near field. The GH effect is the apparent shift of the beam, which produces an AF error.
In other words, if δ is the phase change on reflection, and θ is the angle of incidence, then the GH effect arises from the dependence of δ on θ. (In fact, the GH effect and the related AF error can be considered to be proportional to the derivative of δ with respect to θ, at least to a very good approximation.) This apparent image shift is also produced by an autofocus system that images a source “object” (e.g., a slit or fringes) onto the substrate surface at a glancing angle of incidence and then relays the image to a detector. The position of the image on the detector will depend not only on the height of the subject surface, but also on variations in intensity and variations of phase in AF light reflected from that surface (i.e., the apparent position changes according to the local GH effect). In an AF system, this means that variations in the surficial construction of the substrate, including its previously formed “stack” of multiple thin-film layers and printed circuit patterns, can produce an error in the surface height measurement; this is called the GH error.
A correction of a GH error is called a Goos-Hänchen correction (GHC). An example GHC is discussed in U.S. Patent Publication No. 2011/0071784 (called herein the “'784 reference”). Referring to FIG. 1, AF lights of variable wavelength and polarization (i.e., from a “broadband” source) 50 are reflected from a substrate and detected along with AF light reflected from a reference mirror (item 52). GH effects, assumed to be present, are estimated from measured changes in spectral and polarization properties of the light reflected from the substrate (on which a “stack” of previously formed layers has been formed). For example, the spectral and polarization properties may include a change in the change of phase with respect to the angle of incidence (and position on the substrate). The GH estimates thus obtained are used to estimate corresponding GHC's (item 58).
However, pattern effects are poorly addressed, if at all, in the '784 reference; rather, there is a presumption that the measured changes are due to GH effects (item 54). The AF system's estimates of substrate topography may have errors due to the pattern(s) previously formed on the substrate. These pattern errors arise because the reflectance pattern is overlaid on slit or fringe images, which produces errors in the estimates of their position.
The '784 reference uses beams of AF light produced by broadband sources 50 and reflected from the substrate to detectors. Initial determinations of substrate z-position may include variations in wavelength and phase spectra of the beams (items 52, 54). The initial determinations are used for determining corresponding GHC estimates (item 58) using either an “analog” or “digital” approach. The analog approach is based on the presumptions that changes in wavelengths and phase angles of AF light are generally associated with correspondingly different amounts of GH effect, and that changes in GH effect occurring over the substrate surface can be estimated from these changes in wavelength and phase spectra. Based on the determined GHC's, the wavelength and polarization spectra are adjusted (arrow 64) to “compensate” for corresponding errors presumed to have been introduced to the spectra by the GH effect. In the digital approach, spectral and phase components of the reflected beam are detected separately, and GH effects are estimated therefrom (item 58). The GH effects are used to determine corresponding GHC's that are applied in software to a filter in the illumination system (arrow 64). The GHC's can be used to apply a “correction” to an initial determined substrate position (items 60 and 62). The filter selects respective “corrective” wavelengths and/or polarizations aimed at reducing GH error. Adjusting the wavelength and polarization dependence of the GHC may be done by simulation, followed by an optimization step. The result is a series of coefficients multiplying GHC terms representing contributions from different wavelengths and polarizations. Unfortunately, the corrections thus determined have substantial inaccuracies, and there is currently no way in which to ascertain the reliability of the corrections.
In U.S. Pat. No. 7,940,374 and in Kahlenberg et al., “Best Focus Determination: Bridging the Gap Between Optical and Physical Topography,” Proc. SPIE 6520 (2007), no attempt is made to measure or predict GHC's. Rather, AF measurements are made using a conventional optical AF sensor and a physical sensor (e.g., an air gauge or profilometer), and empirical corrections to AF are determined from these measurements. No direct GHC's are possible in areas of the substrate where both optical and physical sensors are not used. Correcting the optical sensor is not contemplated or performed; rather, data obtained by the optical sensor and by the physical sensor are used to produce an empirical “correction” map. Corrections for a selected chip region (die) are averaged over all dies (having the same pattern) on the substrate to produce an approximate “global” correction. The validity of the “global” correction is assumed to extend to all dies (having the same patterns) on other substrates as well. This method does not take into account the GH effects of actual thin-film variations existing between dies (e.g., middle regions versus peripheral regions) or between respective regions of individual dies.
True GHC provides respective region-specific corrections for various regions of the substrates, taking into account the actual patterns and other GH-specific features in each region. This is important because the GH error can change substantially with, for example, very small changes in thin-film thickness.
The approach described in the '784 reference can be distinguished from the approach in the '374 reference because the former in principle allows estimation of the GHC, even in situations in which physical-sensor measurements are lacking. However, the success of the GHC predictions depends on the actual properties of the patterned wafers (which are not taken into account) and may vary substantially from one location and/or condition to another. The '374 reference does not disclose either measurement or estimation of the GH effect.
The state of the prior art discussed above reveals an existing need for, inter alia, improved methods for correcting (or at least reducing) errors in GH corrections (GHC's) so that autofocus determinations can be made at a speed and accuracy currently demanded for microcircuit fabrication. There are various situations in which conventional GHC schemes as summarized above break down and do not provide adequate correction of the GH effect. An example situation is the presence of unusual pattern features in a particular location on the substrate that are not taken into account in GHC estimations made using prior-art methods.